The present invention relates generally to controlling states of dynamic systems. A particularly well-suited application of this technology is the dynamic control of cranes. Specifically, the present invention can be used to improve positioning capability of cranes and reduce undesirable oscillation of the payload.
Cranes occupy a crucial role within industry. They are used throughout the world in thousands of shipping yards, construction sites, steel mills, warehouses, nuclear power and waste storage facilities, and other industrial complexes. The significant role that these systems maintain in the world can hardly be overestimated.
Cranes are highly flexible in nature, generally responding in an oscillatory manner to external disturbances and motion of the overhead support unit (e.g., the bridge or trolley). In many applications this oscillation has adverse consequences. Swinging of the payload or hook makes precision positioning time consuming and inefficient for an operator. When the payload or surrounding obstacles are of a hazardous or fragile nature, the oscillations may present a safety hazard as well.
The broad use of cranes, coupled with the need to control unwanted oscillations has impelled a large amount of research pertaining to the control of these structures. Broadly, engineers have sought to control three aspects of crane systems, namely, motion-induced oscillations, disturbance-induced oscillations, and positioning capability. These aspects of crane systems are important because the ease-of-use, efficiency, and safety of crane systems can be significantly improved if controlled successfully.
A variety of techniques have been developed for controlling the dynamic response of cranes. Fang et al., in “Nonlinear Coupling Control Laws for a 3-DOF Overhead Crane System,” presented at 40th IEEE Conference of Decision and Control, Orlando, Fla., USA, 2001, proposed to control final trolley position and cable sway through a proportional-derivative type control, in which the coupling between the cable angle and the motion of the trolley is artificially increased. Kim et al., in “A New Vision-Sensorless Anti-Sway Control System for Container Cranes,” presented at 38th IAS Annual Meeting, Industry Applications Conference, 2003, implemented a pole-placement strategy on a real container crane to control cable sway, as well as final positioning. Moustafa in “Reference Trajectory Tracking of Overhead Cranes,” Journal of Dynamic Systems, Measurement, and Control, vol. 123, pp. 139-141, 2001, used nonlinear control laws for payload trajectory tracking based on a Lyapunov stability analysis. Finally, Fliess et al., in “A Simplified Approach of Crane Control Via a Generalized State-Space Model,” presented at 30th Conference on Decision and Control, Brighton, England, 1991, proposed a linearizing feedback control law for a generalized state variable model.
These feedback control schemes are well suited to precisely position the overhead support unit of a crane. However, a difficulty associated with feedback is related to multi-state control. When a feedback controller must minimize cable sway, in addition to positioning a bridge or trolley, the control task becomes much more problematic. Accurate sensing of the payload must be implemented, which is often costly or difficult. When sensing of the payload is available, the control does not respond unless cable sway is present. In this way, the control is inherently reactive instead of anticipatory.
Time-optimal control is a common open-loop approach for obtaining swing free motion. One of the drawbacks to many time-optimal control schemes is their inability to be implemented in real-time owing to the necessity of precomputation of system trajectories. As was indicated by Gustafsson et al., in “Automatic Control of Unmanned Cranes at the Pasir Panjang Terminal,” presented at 2002 IEEE International Conference on Control Applications, Glasgow, Scotland, U.K., 2002, there is no known implementation of a time-optimal control scheme used with a commercial crane.
Several patents relating to crane control have been issued. These include U.S. Pat. No. 4,756,432, issued Jul. 12, 1988 to Kawashima, et al., U.S. Pat. No. 5,526,946, issued Jun. 18, 1996 to Overton, U.S. Pat. No. 6,050,429 issued Apr. 18, 2000 to Habisohn, U.S. Pat. No. 5,908,122, issued Jun. 1, 1999 to Robinett, et al., U.S. Pat. No. 4,997,095, issued Mar. 6, 1991 to Jones, et al., U.S. Pat. No. 5,529,193 issued Jun. 25, 1996 to Hytonen, U.S. Pat. No. 5,127,533 issued Jul. 7, 1992 to Virkkunen, U.S. Pat. No. 6,102,221, issued Aug. 15, 2000 to Hibisohn, U.S. Pat. No. 5,938,052, issued Aug. 17, 1999 to Miyano, et al., U.S. Pat. No. 5,785,191, issued Jul. 28, 1998 to Feddema, et al., U.S. Pat. No. 5,960,969, issued Oct. 5, 1999 to Habisohn, U.S. Pat. No. 5,961,563, issued Oct. 5, 1999 to Overton, and U.S. Pat. No. 5,909,817, issued Jun. 8, 1999 to Wallace, Jr., et al.
The present invention addresses the drawbacks and limitations of many of the aforementioned control schemes. Specifically, simultaneous real-time positioning, motion-induced oscillation suppression, and disturbance rejection of cranes is achieved in an easily implementable and computationally simple control scheme.